1. Technical Field
The present invention relates to a method for manufacturing single-crystal silicon, and particularly to a method for manufacturing single-crystal silicon manufactured by a Czochralski method.
2. Description of the Related Art
The Czochralski method has been widely used in the manufacture of single-crystal silicon. In the Czochralski method, a polycrystalline material is melted in a quartz crucible, and a seed crystal is brought into contact with a material melt and is pulled up, thereby growing the single-crystal silicon.
Dislocation of the single-crystal silicon occurs in some cases during growth of the single-crystal silicon. It is pointed out that one of the reasons for dislocation of the single-crystal silicon is that cristobalite precipitated on an inner surface of the crucible made of quartz glass is released into the silicon melt and the cristobalite is taken into a silicon ingot when the silicon is pulled up.
Japanese Patent Laid-Open Application No. 9-110590 describes a method for promoting crystallization of quartz and preventing release of cristobalite by adhering a devitrification promoter containing an alkaline-earth metal such as barium to an inner surface of a quartz crucible.
Japanese Patent Laid-Open Application No. 2003-160393 describes a method for reducing occurrence of dislocation during growth of single-crystal silicon by decreasing the concentration of a devitrification promoter, which is adhered to an inner surface of a quartz crucible, at a portion where the temperature during pulling-up of the single-crystal silicon is high, and increasing the concentration of the devitrification promoter at a portion where the temperature is low.
In recent years, single-crystal silicon has been manufactured using a multi-pulling method for pulling up a plurality of pieces of single-crystal silicon from a material melt in an identical crucible. According to this method, a single crystal is pulled up from the material melt, and then, a polycrystalline material is additionally charged into and melted in the remaining material melt without turning off a heater, and then, a next single crystal is pulled up. By repeating these steps of additionally charging the polycrystalline material into the material melt and melting the polycrystalline material in the material melt, and then, pulling up a next single crystal, a plurality of pieces of single-crystal silicon is pulled up using the identical crucible.
When the plurality of silicon single crystals are pulled up in the identical crucible using the above-mentioned multi-pulling method, the operation takes a long time, and thus, the crucible is also exposed to high temperature for a long time. Even if a crucible having a denitrification promoter containing barium adhered thereto is used in the manufacture of the single-crystal silicon by the multi-pulling method, dislocation occurs in some cases when the single-crystal silicon is separated from the material melt.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing single-crystal silicon that makes it possible to reduce occurrence of dislocation of the single-crystal silicon when the single-crystal silicon is grown using the multi-pulling method for pulling up a plurality of pieces of single-crystal silicon from a material melt in an identical crucible.
As a result of earnest study, the inventors of the present invention have found that occurrence of dislocation of single-crystal silicon is affected by an amount (dose amount) of added barium on an inner wall surface of a crucible. When the amount (dose amount) of added barium is large, the rate of occurrence of dislocation of the single-crystal silicon is high. On the other hand, when the amount (dose amount) of added barium is small, the rate of occurrence of dislocation of the single-crystal silicon is low. When the amount (dose amount) of added barium becomes smaller than a certain amount (dose amount), the rate of occurrence of dislocation of the single-crystal silicon becomes high.
The reason why the rate of occurrence of dislocation of the single-crystal silicon is high when the amount (dose amount) of added barium is large is considered as follows. First, cristobalite is produced on the surface of the quartz crucible by a denitrification promoter adhered to the surface of the crucible and containing barium. When single-crystal silicon is pulled up to a certain length from the silicon melt, and then, the single-crystal silicon is separated, silicon solidifies before cristobalite released into the silicon melt flows down from a tail part to the silicon melt, and the cristobalite is taken into the silicon as an impurity. As a result, dislocation occurs in the single-crystal silicon and the silicon is dislocated.
On the other hand, when the amount (dose amount) of added barium is extremely small, the rate of occurrence of dislocation of the single-crystal silicon is high. One of the reasons for this is considered as follows. When the amount (dose amount) of added barium is small, uniform crystallization of quartz on the inner wall of the crucible is not achieved, and thus, the cristobalite is released into the silicon melt and taken into the single-crystal silicon.
When the crucible has a large size, the temperature of the crucible during growth of the single-crystal silicon is high. High temperature of the crucible promotes crystallization. Therefore, the amount of barium melted into the silicon melt varies depending on the size of the crucible.